We propose to study the molecular mechanisms that control transcription of ribosomal RNA in the frog, Xenopus. The study can be divided into three sections. 1) Control regions of ribosomal genes will be sequenced and the nucleotides that interact with regulatory molecules will be determined. Control regions will include promoters and terminators for RNA polymerase and they will be determined for both X. laevis and X. borealis. 2) Work will continue on development of an in vitro transcription system capable of accurately transcribing added ribosomal DNA. Considerable progress has already been made toward this goal using homogenates of manually isolated oocyte nuclei. 3) The molecular mechanisms controlling ribosomal RNA synthesis will be investigated in several biologically interesting situations. These are: the repression of rRNA synthesis during early frog embryogenesis; the repression of borealis rRNA transcription in laevis-borealis hybrid embryos; and the reduction in rRNA synthesis during amino acid starvation of cultured cells.